This invention relates to a control apparatus for a synchronous motor, and more particularly to a positioning control apparatus for a synchronous motor.
A positioning control apparatus for a synchronous motor is disclosed in detail by a U.S. Pat. No. 4,051,419, entitled "CONTROL SYSTEM OF AN ALTERNATING-CURRENT MOTOR" owned by the same applicant of the present invention. But the positioning control apparatus for a synchronous motor disclosed by the U.S. Pat. No. 4,051,419 belongs to a closed-loop control system in which the feedback signal is obtained from the instantaneous angular position of the rotor of the motor under control. For some applications, however, an open-loop positioning control system is more desirable, and a stepping motor or a so-called pulse motor has exclusively been used in the heretofore known open-loop positioning control system.
As is well known, a stepping motor has several disadvantages. One disadvantage of a stepping motor is that the motor experiences a vibratory transient at each input pulse, since the motor is rotated by such a large amount of an electrical angle as 180.degree. for each input pulse.
Another disadvantage of a stepping motor is that the motor has not a sufficiently large torque to retain an angular position when the input pulse to the motor is in a quiescent state.
Still another disadvantage is that the heretofore known stepping motor control system lacks the capability of recovering a lost step when the motor failed, by some mischance, to move a step for an input pulse.
It is, therefore, a principal object of the present invention to provide a control apparatus whereby an open-loop positioning control is attained by a synchronous motor and whereby all the demerits which are inherent to a stepping motor are eliminated.
A further object of the invention is to provide a control apparatus, whereby the respective magnitudes of the currents flowing in the stator windings of a snychronous motor are controlled in such a way as the resultant magnetic field generated by these currents (hereafter will be called a stator magnetic field) is in a direction corresponding to the reference angular position .phi., at which the rotor of the snychronous motor is positioned.
Another object of the invention is to provide a control apparatus in which the respective magnitudes of the currents flowing in the stator windings of a synchronous motor are controlled by chopping a constant d. c. voltage and regulating the current flow angles in proportion to the respective reference magnitudes of the currents.
A further object of the present invention is to provide a control apparatus in which the respective reference magnitudes of the currents in the stator windings of a synchronous motor are obtained from a reference angular position .phi..
Another object of the present invention is to provide a control apparatus in which a digital code representing a reference angular position .phi. of a synchronous motor under control is stored in a pulse counter as an integrated value of the number of input pulses to the counter.
These and other objects of the present invention will become manifest upon a study of the following description and the accompanying drawings.